The ability to ionize gases is useful for a wide range of applications including many chemical detection applications. Ionization techniques, in which a gas sample is ionized and then separated into constituent parts that can be detected individually, are widely used for gas composition sensing. Two well-known examples are Ion Mobility Spectrometry (IMS) and Field Asymmetric Ion Mobility Spectrometry (FAIMS), also known as Differential Mobility Spectrometry (DMS). Ion mobility detection techniques tend to be very well suited to measuring trace constituents of gas mixtures that often consist of a carrier gas with additional gases mixed in at low concentrations (for example part-per-million or part-per-billion levels). Ion mobility techniques can also be used effectively over a range of gas pressures, including pressures close to one atmosphere. This makes them useful for, amongst other things, measuring low-level impurities in air. Because they work by measuring properties of ionized molecules and because gas samples for analysis generally consist mainly of neutral molecules, ion-mobility-based detectors generally incorporate an ionizer. The sample gas is passed through the ionizer to produce a population of ionized molecules that are then manipulated in some way involving separation or selection of ionized molecules according to their behavior in an electric field, before being detected. Ionizers commonly in use include radioactive sources, light-based devices such as ultra-violet lamps, and electrostatic devices such as corona discharge ionizers.
Radioactive sources have long been used as ionizers for chemical detection systems. It is noted radioactive isotopes such as 241Am or 63Ni are commonly used as ionization sources to generate ions in a surrounding gas stream. Advantages of radioactive sources as ionizers include stable and well-understood ion chemistry and the ability to ionize without an external power source. A major drawback, however, is that radioactive sources pose a health hazard and are therefore not suitable for use in many applications and are subject to strict government regulation. Non-radioactive ionizers, including corona discharge ionizers, do not suffer from this disadvantage and can be widely and safely deployed in a range of applications.
A commonly used ion source in the field of chemical detection is the radioactive isotope 63Ni. The interactions of ionizing radiation emitted by 63Ni with many types of gas molecules have been studied and understood, meaning that the ion species produced when a 63Ni source ionizes a gas mixture of a given composition can generally be predicted with high confidence. 63Ni can therefore be thought of as a “reference” ionizer for many gas detection systems. Radioactive ionization sources have the advantage of simplicity, compactness, durability, and reliability. The regulations associated with these radioactive ionization sources, however, may render the incorporation of radioactive isotopes into a product commercially unfeasible. Therefore, there exists a need for an ionizer that has similar ionization properties to 63Ni but that does not suffer from the safety and regulatory drawbacks associated with radioactive sources.
It is also to be appreciated that electric field ionization has the advantage of a relatively simple design, relatively simple fabrication, and low power consumption. For instance, in electric field ionization, a large electric field typically between 107 to 108 V/m is generated between two electrodes. The large magnitude of the electric field accelerates any ions or other free charges within the field thereby causing the accelerated ions or other charges to collide with surrounding gas molecules. The collision of an accelerated ion or other charged particle (such as an electron) and a gas molecule creates an ionized molecule. A corona discharge is a type of electric field ionization where a neutral fluid such as, for example, air is ionized near an electrode having a high electric potential gradient. Such a potential gradient is achieved by using a discharge electrode having a small radius of curvature. The polarity of the discharge electrode determines whether the corona is a positive or negative corona. Typically, the corona has a plasma region and a unipolar region. In the plasma region, electrons avalanche to create more electron/ion pairs. In the unipolar region, the slowly moving massive (relative to the electron mass) ions move to the passive electrode, which is usually grounded. If the plasma region grows to encompass the passive electrode, a momentary spark or a continuous arc may occur. The spark or arc may damage the electrodes, produce contaminant ions, and reduce the lifetime of the ionization source. Therefore, there remains a need for devices and methods providing improved ionization.